Register a phone number

Sends a one-time code by SMS to the phone number that will own the new account. Call this with the `registrationId` returned by [Verify a sign-up code](#operation/verifySignup). The number must be a valid phone number in E.164 form, and it must not already be registered to another account — **a verified number is bound to one account permanently.** A Dial number cannot be used to register a Dial account. Calling this again with the same number re-sends the code. Calling it with a different number replaces the pending one, so a mistyped number is recoverable until it is verified.